Turbofan gas turbine engines are used for powering aircraft and comprise a relatively large diameter fan, which is driven by a core engine. The fan is vulnerable to damage as a result of foreign objects entering the turbofan gas turbine engine. In most cases, the fan is sufficiently robust to withstand the effects of such foreign object ingestion without suffering major damage and is able to continue operating, although, perhaps, at reduced efficiency.
On very rare occasions, the fan may be damaged to such an extent that parts of one or more of the fan blades that make up the fan are lost. This usually necessitates shutting down of the turbofan gas turbine engine involved to minimise the hazard to the aircraft carrying it. However, the imbalance in the fan created by the fan blade loss initially generates extremely high loads, which must, at least partially, be absorbed as the gas turbine engine is allowed to run-down to windmilling speed. Windmilling speed is the speed at which the gas turbine engine rotates in a non-operative condition as a result of its motion through the atmosphere.
The transients following the fan blade loss produce massive loads and distortion of the bearing housing for the fan bearing and also in surrounding structure.
One way in which the fan imbalance load absorption may be achieved is to ensure that the relevant engine structures are sufficiently strong to tolerate the very high loads involved.
However, this results in a heavily reinforced structure both in the engine and aircraft, which results in an increase in weight of the engine and aircraft.
Other ways in which the fan imbalance load absorption may be achieved is to provide energy absorbing links and deforming housings.
Again, this results in an increase in weight of the engine and aircraft and the movement of the energy absorbing links or deforming housings results in permanent deformation of the structure and does not give a stiff structure to control shaft/rotor dynamics during windmilling.